For purposes of geologic exploration, such as for example in connection with exploring the earth for underground petrochemical fossil deposits, it is known to measure certain electrical properties existing along an earth-formation wall. For example, the variations in the electrode currents and/or voltages thus monitored and displayed have been found to correlate with certain properties of the adjacent earth formation. One way in which this correlation can be determined is to form a hole in the earth by coring. For example, a cylindrical cut can be made into the earth and the core inside the cut removed intact, to leave a borehole. Logging of the exposed wall inside the borehole can then be performed, and the resultant measured electrical values at each depth may be compared with the structure and properties of the core at corresponding locations. Such coring and logging can enable one to attribute specific physical, chemical, and geologic properties to the logging data obtained, with a high degree of certainty.
A borehole can be formed, for example by a conventional rig, and with the borehole filled with a drilling mud, an electrode structure on a pad is moved along the length of the borehole while a voltage is applied between the measurement electrodes on the pad and a return electrode. The resultant currents and/or voltages present at the borehole wall are monitored and recorded. Recording may be on a magnetic tape, on an ink recorder, on a computer disk or in a computer memory, as some examples. A simultaneous record of the position of the electrode structure along the borehole wall can be made, so that a plot of distance along the borehole against a selected electrical property may be formed from the data obtained, from which a visually-observable image of the segment of the borehole wall swept over by the electrodes on the pad can be generated.
As one example, a technique for obtaining such electrical measurements of a formation is to use an array of electrodes or “buttons” mounted on, and insulated from, an electrically conductive pad. The pad is pushed against the borehole wall as the pad is moved along the borehole, while a voltage or current is applied between each button and another return electrode, such as a remote electrode located on the tool body. The currents flow at least in part from the electrodes through the borehole wall, in a magnitude dependent upon the properties of the earth formation adjacent the electrodes. Such procedures have been termed as microresistivity measurements, since they measure the electrical resistivity of relatively small vertical segments of the wall structure.
Improvements may yet be made to the acquisition of electrical data from the borehole logging during investigation of formation properties, including for example that are less complicated and less time consuming, and that may be viable with existing well-logging tools and systems, and that may be viable for example with bandwidth capabilities of such tools and systems.